There are many situations wherein it is desired to avoid exposure of a liquid to the atmosphere and also to maintain a minimal air or gas pocket between the liquid surface and the seal. This is particularly desirable in the wine making industry, for example, where the volume of liquid to be sealed varies as liquids are added or removed. An air space above the wine will cause an increase of oxidation within the upper levels of the liquid. One solution of this problem has been to purge the area between the liquid surface and a sealed lid with an inert gas such as nitrogen. This process proves to be tedious and expensive, especially when the liquid volume fluctuates often.
Another solution has been to provide the holding tank with a lid that is vertically movable within the tank to follow the fluid level. Both floating and manually movable lids have been used with varying degrees of success.
The difficulties with manually movable lids is the tedium and possibility of oversight in moving the lid as liquid volume changes. Floating lids move automatically as the liquid volumes change and so eliminate the need for manual movement of the lid. The drawback here is that floating lids are often not effective as pressure seals and often allow interchange of gases from both sides of the lids. This is undesirable in instances, especially in wine making, where exposure of the liquid to the atmosphere is undesirable and where the fermentation process is kept in check by regulating pressure within the vessel.
U.S. Pat. No. 2,014,246 discloses a sealing device for storage tanks wherein a hollow core lid is shown floating on the surface of a liquid in a cylindrical tank. FIG. 3 of the drawings in this patent shows rigid top and bottom plates spaced by a peripheral inflatable seal. The plates are substantially horizontal and, as such, will not form a "pocket" of collected gases away from the peripheral rim. Furthermore, the rim itself is also used as the connecting wall between the vertically spaced plates. The flexible area therefore extends the full axial distance between the top and bottom plates. Special precautions must be taken to secure the seal to the plates.
U.S. Pat. No. 1,861,868 to D. E. Larson discloses a floating roof seal that makes use of a plurality of cylindrical buoyant sealing members that are urged against the upright side walls of the storage tank and against a conical surface of the lid to effect somewhat of a seal between the liquid and the atmosphere. Obviously, air spaces between the individual sealing members will allow at least some seepage of air or gases between the liquid and atmosphere.
A complex sealing arrangement using an inflatable rim is disclosed in U.S. Pat. No. 2,538,875 to Laird. Laird discloses a single flexible membrane spanning the surface of a liquid in a tank. This membrane is attached to an actuator valve that controls inflation of an annular seal. Inflation of the seal is dependent on deflection of the flexible cover.
U.S. Pat. No. 4,071,164 discloses a collapsible lid arrangement having an annular seal or serrated edge for engaging the inside walls of a spherical tank.
U.S. Pat. No. 1,650,340 is illustrative of a "floating" tank member that makes use of a series of overlapping inflatable tubes to produce a continuous seal around the lid periphery.
U.S. Pat. No. 2,664,220 is illustrative of a sliding seal for a "floating roof construction". Again, a single plate spans the liquid surface and an inflatable seal member as indicated in FIGS. 3 and 4 of the drawings are used to seal the liquid from the atmosphere.
U.S. Pat. No. 1,735,461 shows an inflatable seal for a single plate floating tank roof. This patent discloses use of a peripheral inflatable tube as a seal with the shape of the tube being alterable by a float situated within the liquid below.
U.S. Pat. No. 4,067,476 is illustrative of a mechanical sealing arrangement for liquid in a cylindrical tank. Here, instead of an inflatable tube, a toggle mechanism is used to press a sealing gasket against the tank side wall.